1. Field of the Invention
The present invention relates to an image processing apparatus and method for converting the gradation of an image, and to a program and a storage medium for realizing such an apparatus and method.
2. Description of the Related Art
Histogram equalization is known as a conventional method for emphasizing the contrast of an entire image. In this method, a gradation conversion function is determined so that the distribution of a histogram after performing gradation conversion is flat (uniform) (For a detailed algorithm, refer to “Digital Image Processing: by Rosenfeld and Kak, translated by Makoto Nagao”).
An automatic gradation conversion apparatus utilizing histogram equalization has been disclosed in Japanese Patent Publication No. 2808773. The gradation conversion function is also called a gradation conversion curve. In actual gradation conversion, a look-up table conforming to the gradation conversion function is mostly used.
Conventionally, X-ray images have been mainly film images. Recently, however, the use of digital X-ray images obtained by digital X-ray photographing systems using a large area sensor, CR systems using stimulable phosphors, or the like is in progress. In such X-ray digital images, doctors tend to desire images equivalent to film images because they are accustomed to diagnose using conventional film images. Accordingly, an image equivalent to a film image has been obtained by performing gradation conversion processing using the characteristic curve of a film as a gradation curve.
However, in gradation conversion processing according to histogram equalization, the gradation curve is determined only by flattening of the histogram of an image after conversion. Accordingly, the smoothness of the gradation curve is not guaranteed. As a result, an X-ray digital image frequently becomes an image different from that of a conventional film image. Such an image is difficult for doctors, accustomed to diagnose using conventional film images, to perform exact diagnosis.